I just ytried that exercise- managed to do it a couple of times and it was already getting difficult.
I've used dexterity putty before - thats quite good.
But it does come with practice.
I'd been playing for a while and then I tried to reach the E/G# chord (422100) and wasn't even close. Now, my pinky can stretch further and it's no problem.
It's a bit of a no pain, no gain situation tho!
JE